Saturday, March 28, 2009

Potentially Inappropriate Posts Part 1: Human Sexuality, Addendum

Tengu commented on my first human sexuality post and pointed me in the direction of very interesting study asking much the same question I did. Why do human females have perpetually large breasts? The study is illuminating, but leaves a few questions unanswered, though I encourage you all to read it. First of all, the authors argue that large breasts got women in hunter/gatherer societies through tough times--they basically acted like camel humps (fat stores). Well, here's my question: why the breasts? They offer tenuous evidence that breast fat somehow converts to milk, but I'd like to see more on that. The human body stores fat in all sorts of places: thights, butt, belly, and breasts. But why would natural selection drive fat stores for women only to the chest?*

Second, it large breasts were so important for early societies, why aren't they strongly written into modern human genomes? The fat-store theory is great, but it implies a pretty strong evolutionary bottleneck--you did not survive the dry season without big breasts. So you get to modern society and women like Amanda Beard** (an olympian swimmer) crop up. Again, I voice a question I asked before: If large breasts confer any kind of advantage, be it for sexual selection or natural selection, why aren't large breasts the standard?

Although, I guess I can't know that for sure. Somebody would have to run one helluva study to figure out the "average" breasts size among various populations of women. Would that average change across traditionally "racial" or culture boundaries? I just got a cool book which features Playboy centerfolds for 54 years, from 1953-2007, and intend to figure out the average myself, leaning on the assumption that Playboy is an accurate barometer of the "American Girl." What I'm finding difficult here is that measurements are not always known and are sometimes simply incorrect, so...yeah***. This is a story for another day, though.****

I don't really see a reason why large breasts can't be the result of both sexual selection and natural selection. It would be advantageous for men to find a naturally selected feature attractive, as it would more quickly disperse that trait among the gene pool. Look at the whole mess of display organs in animals. Dimetrodon has a giant dorsal sail that probably functioned as a thermoregulatory device but could have easily functioned for sexual signaling. Modern giraffes have long necks for both reaching high vegetation (and thus neatly avoiding competition) and battling with rivals. Triceratops had horns and frills that have been shown to have intraspecific display/battle functions, but you can damn well be sure that it's gonna defend itself against a marauding tyrannosaur with that same weaponry. A single structure can have multiple uses.

And another reason why small breasts might successfully "infiltrate" any sort of norm could be just the fact that our genes are spread extremely far, and a whole host of new phenotypes have popped up as a result. And thanks to the same mechanism, no matter what the phenotype, somebody will find it attractive (on a man or a woman), so it successfully remains in that population's gene pool. Anyway, go ahead and read the article, and post your thoughts below!

*And if large breasts are such an advantage during the dry season, why wouldn't other non-human primates develop them?

**Amanda Beard might be a poor choice. Atheletes tend to lose every inch of body fat, especially olympians. Ms. Beard might have had larger breasts before diving into the pool. She's just the only example I could come up with on a whim.

***Playboy likes to pretend the breasts don't get much larger than a DD. There are only four, maybe five examples of playmates with cited busts of DD, and I question whether these measurements are actually too conservative. I could list some names, but this is honestly a topic for an entirely separate post.

****Yes, my wife is aware of the book. She thinks I could publish such a study in a social sciences journal.


Noadi said...

Here's my theory on this: My understanding is that a good bit of breast size is also affected by body fat during puberty. Large breasts in a young woman in a hunter gatherer society would indicate health and a family that provided well. Such a woman would be a valuable mate both because she's healthy and because her family would be good allies to have. Selection would then favor women with larger breasts.

Metalraptor said...

The real question is, why are we the only primates who did develop them. If large breasts provide both a sexual and biological advantage, whether it be fat storage, sexual display, or whatever, seem to provide an advantage to humans that other primates simply do not have (perhaps breasts may have something to do with humans being so widespread, and other non-Homo sapiens apes being very localized or extinct. If breasts are so advantageous, why are we the only ones who have them. Wouldn't there be more animals out there with permanently enlarged secondary sexual organs?

Zachary said...


Christopher Taylor said...

There was Desmond Morris' suggestion in The Naked Ape (yay, I'm quoting twaddle!) that rounded breasts developed as part of the transition from rear-mounting in other primates to face-to-face copulation in humans. His suggestion was that they mimicked rounded buttocks.

Two main problems that I can see with that: (1) Rounded buttocks are also a uniquely human feature (other primates go in for genital swellings), so why should we assume that they developed before large breasts? (2) Morris is betraying his Euro-centrism again, I fear. Rounded breasts are more a European feature, while the more elongate breasts common in African countries aren't quite so "butt-like".

The differences in copulating positions could go some way to explaining why humans have developed such a strong ventral display but other primates haven't. But don't bonobos also regularly copulate face-to-face? How do bonobo secondary sexual characteristics differ from chimpanzees'?

Brad said...

Playboy centerfolds as an unbiased representation of variation in American women? Are you kidding??? Playboy likes to pretend the breasts don't get much larger than a DD because they also like to pretend that all women weigh about 110 lbs.

Actually, I think there already have been a few papers in the social sciences on centerfold statistics, but I forget what the researchers were trying to find out and I don't feel like looking it up.

Metalraptor said...

The whole face-to-face sex doesn't really explain anything, as Chris stated below. It does smack of euro-centrism, and the fact that bonobos and other apes mate face to face don't help at all. The other question is why didn't we go for the swelling of buttocks? Many other primates did, and yet we didn't.

And then we can get into the whole debate on why all animals don't have junk in the trunk, because it is sexually advantageous (if not as advantageous biologically, as baboon females if they don't mate have their butts swell even bigger the next time).

BusaFan said...

Here's a crazy thought. Maybe "abnormally large" breasts... aren't. Couldn't these same arguments be used for abnormally tall people? There are a lot of perfectly healthy people in the 6'4" to 6'8" range. Probably about the same amount percentage-wise as women with all natural DD breasts. Just a guess though.

For every woman with DD or larger breasts, there is a woman with As, or virtually no breast at all. (same difference I suppose)

Therefore large breasts are no more advantageous that a lack of large breasts. It's simply the normal variation of a given population.

Natural Athlete said...

The first question is why breasts in the first place women in all human population have some degree of fat depositing in the mamary area. This is unique to human beings. I tend to think that this and the fact that breasts are fascinating to almost all heterosexaul men even those who perfer buts and many women to indicates some degree of sexaul selection. As to the exact why of breasts I think the argument that upright posture and importance of face to face communication made a ventral sexaul display valuable is pretty legit. Why it was breasts and not red pocky a dots is one of the questions like the peacock tail or the lions mane evolution works with what it has. Humans carry more subcutaneous body fat then other primates it has to go somewhere putting in the mamary area seems to have worked well for women and men ended up liking it. Women have fat deposits are their but and hips that other primates don't have and men like those too. So I think there is combination of natural and sexual selection here too.

Bonobos have their own exaggerated secondary sexual characteristic they have permanently enlarged labial lips. In other primates this only occurs during estrus and then for mos species not as exaggerated as in bonobos with some notable exceptions.

As far as why variation arises in breast size maybe large breasts are on sexual strategy were as maybe a well deloped posterior is possible competing strategy. I have seen some evidence that their differences in hormonal profiles in women who have large breasts vs those with large butts and also in the profiles of the men who prefer one or the other.

Reverend Courtney said...

I would think that the fat stores theory is more due to the body's ability to get a child, not the mother through the winter.

I think that only women have them because of a link to milk production and child rearing.

Mostly, my theory is this:
Women have developed and maintained a large breast size, because, they, much like plumage or other gender identification factors, can be seen from a distance; and the bigger they are, the easier they are to identify, that is, the easier a potential sexual rendevouz is to identify, both from a distance, and also from beneath, heavy skins, cloaks, and clothing. So..
Men can easily recognize potential sex, and possibly companionship, and in ancient times, avoid entanglements with competing males, by identifying women by their breasts.. Thus, sexually, especially, during the ice age and since the advent of marketing, these specimens of our species were bred, and traits passed on.. I am not sure if this would be considered natural selection, but that, I suppose is the angle that I am approaching it by. So, regardless of whether or not, the physical genetics of "large" breasts have been passed on in our genome, the male brain sees large breasts as an instant sign of potential sexual gratification if nothing else..
I am curious as to whether there are any studies to back this up.
I have also read about the mentioned study, where the breasts were said to resemble the roundness of buttocks as we became bipedal. And although the comment mentioned that swollen genitalia is typically what is sought by primates and that this somehow ruled out the attraction to rounded buttocks, I disagree, if primates look for swollen genitalia, they are aware of their proximity to the buttocks, or surely the human mind is, and visually, what better way to physically embody that need to find swollen genitalia, than buttocks, which are round, and themselves represent the swollen genetalia... well, anyhow that's my 2 cents plus some ..